V-belt CVTs are widely used in small vehicles, such as straddle-type vehicles including motorcycles, four-wheeled buggies and snowmobiles. The V-belt CVT includes a primary shaft to which torque is input from a power source such as an engine or an electric motor, a secondary shaft from which the torque is output to a driving wheel, and a primary sheave and a secondary sheave with a variable groove width respectively disposed on the primary shaft and the secondary shaft. A V-belt is wound around the primary sheave and the secondary sheave. The speed change ratio between both the sheaves is adjusted continuously by changing the winding diameters of the V-belt around the respective sheaves, by varying the groove widths of the sheaves through a groove width adjustment mechanism.
In general, the primary sheave and the secondary sheave each includes a pair of a fixed flange and a movable flange disposed oppositely for movement toward and away from each other along the primary shaft or the secondary shaft. A groove in the shape of the letter “V” (which is hereinafter referred to as “V-groove”) is formed between the fixed flange and the movable flange. The groove width adjustment mechanism moves the movable flange of the primary sheave and the secondary sheave to vary the width of the V-groove. This causes the winding diameter of the V-belt around the sheave to change, allowing continuous adjustment of the speed change ratio between both the sheaves. A centrifugal governor is commonly provided to the movable flange of the primary sheave as the groove width adjustment mechanism (see Patent Document 1, for example).
In recent years, small vehicles have been required to incorporate an automatic transmission that can achieve suitable speed change ratios according to the running condition and operating state of the vehicle in order to improve the running stability and promote energy conservation. However, the conventional groove width adjustment mechanism employing a centrifugal governor determines the speed change ratio between both the sheaves solely based on the rotational speed of the movable flange provided with the centrifugal governor. Thus, it has been difficult to control the speed change ratio flexibly, for example according to the operating state such as acceleration and deceleration.
In order to solve such a problem, there has been proposed a V-belt CVT provided with a groove width adjustment mechanism that employs an electric motor instead of a centrifugal governor to change the widths of the respective V-grooves of the primary sheave and the secondary sheave, so as to control the speed change ratio between both the sheaves to desired values (see Patent Document 2, for example).    Patent Document 1: JP-B-Sho 63-33588    Patent Document 2: JP-B-2967374